The present invention relates to a voice communication apparatus for receiving voice data by the frame.
An IP telephone set has been known as one of a voice communication apparatus for receiving voice data, in which voices are encoded by a predetermined encoding system, frame by frame. In the IP telephone set, conversation is carried out between the transmission side and the receiving side by packets internally including one or more frames via a network such as the Internet.
In the network environment, loss of a frame that stores voice data occurs due to loss of a packet. Since the loss of the frame becomes a factor in the lowering of voice quality, in a general packet loss compensation (PLC), the lost frame is interpolated based on the voice data of the previous frame normally received.
If a Codec (coder/decoder) that is not influenced by the previous data as in G.711 Appendix I is available, data close to the original voice can be reproduced by using the loss compensation of the Codec.
However, since G.726 is influenced by the previous data, the interpolated lost frame becomes non-continuous to the voice data of a normally received frame thereafter. Therefore, when a normally received frame is decoded, the voice is made into an abnormal impulse-like sound, depending on voice data, and becomes a harsh grating noise to a receiver.
JP-A-8-292799 describes a method for preventing acoustic quality deterioration subsequent to a frame loss that becomes a factor for such a harsh grating noise, although the method relates to a mobile wireless device such as a digital cordless telephone.
JP-A-8-292799 describes an ADPCM encoded voice decoding method that decodes a correction code sequence with respect to the ADPCM (Adaptive Differential Pulse Code Modulation), which carries out correction using control and coded voices for making a predicted differential value small in regard to a part of the code sequence of a frame next to a lost frame.
However, in some lost frames, there are cases where the lost frames do not include abnormal impulse-like noise when encoding a normal frame next to a lost frame, and can have satisfactory reproducibility so as to be able to hear with normal sound quality. Therefore, if, as in the ADPCM encoded voice decoding method according to JP-A-8-292799, voice data is corrected in all cases regardless of whether or not there are abnormal impulse-like sounds in normal frames subsequent to lost frames, there is a fear that correction will inversely bring about acoustic deterioration in quality.